Republican Donald Trump and Democrat Hillary Clinton are headed to their second U.S. presidential debate late Sunday, even as key Republicans are demanding that he drop out of the race after a 2005 tape recorded him making lewd comments about women and boasting how he groped them.

Trump said Saturday there was "zero chance" that he would quit the race for the White House a month before the November 8 election and apologized for his comments.

But Clinton, a former U.S. secretary of state looking to become the country's first female president, has yet to speak in public about Trump's self-proclaimed sexual exploits and is poised to confront him at the debate in the Midwestern city of St. Louis, Missouri. In a Twitter statement, she called his comments "horrific."

Students playing the roles of the candidates go through a rehearsal for the second 2016 U.S. presidential debate at Washington University in St. Louis, Missouri, Oct. 8, 2016.

'A distraction'?

Meantime, Trump, a brash real estate tycoon making his first run for elected office, said the controversy is "nothing more than a distraction," although national political surveys had shown his support slumping even before the existence of the tape became known on Friday. Clinton is holding more than a four-percentage-point advantage and gaining ground in key battleground election states.

​Trump, in his taped apology, suggested that he will attempt to pivot from his bawdy conduct to cast aspersions on Clinton, claiming that she enabled infidelities by her husband, former president Bill Clinton, and disparaged women who alleged that the country's 42nd chief executive had made unwanted sexual advances on them over several decades. Clinton was impeached for lying about an affair with an intern in the White House, but not convicted, and then finished out two terms in office in 2001.

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“I’ve said some foolish things,' Trump said, "but there is a big difference between the words and actions of other people. Bill Clinton has actually abused women and Hillary has bullied, attacked, shamed and intimidated his victims.”

A Trump supporter, former New York Mayor Rudy Giuliani, told CNN Sunday that Trump would not hesitate to talk about "the women that Bill Clinton raped, sexually abused and attacked."

On his Twitter account Sunday, the embattled Trump lashed out at Republican critics who have abandoned him, saying, "So many self-righteous hypocrites. Watch their poll numbers, and elections, go down!"

​ Calls for Trump to drop out

A poll of more than 1,500 voters by the political news site Politico with Morning Consult, conducted after the disclosure of the tape, showed that about 70 percent of Democrats, but only 12 percent of Republicans, think Trump should quit the race.

Several Republican elected officials called for Trump to drop out of the contest to become the country's 45th president when President Barack Obama leaves office in late January. But there was no immediate consensus how he would be replaced on the national ticket even if he were inclined to quit.

Meanwhile, absentee and early voting has already started, with about 400,000 ballots cast in recent days. States across the country have already printed ballots with Trump's and Pence's names listed as the Republican candidates and ballots mailed to military service personnel stationed overseas.

Key Republicans, including his vice presidential running mate, Indiana Governor Mike Pence, condemned Trump's 2005 comments that were recorded on a live microphone while he was on a backlot of a Hollywood set just before a cameo appearance on a television soap opera.

"I do not condone his remarks and I cannot defend them," Pence said as he backed out of a planned campaign appearance Saturday in Wisconsin, where he had been expected to fill in for Trump.

The 2008 Republican presidential candidate, Arizona Senator John McCain, retracted his support for Trump, but stopped short of calling for him to drop out of the presidential race. The 2012 nominee, former Massachusetts Governor Mitt Romney, who like McCain lost to Obama, denounced Trump's sexual braggadocio, saying, "Hitting on married women? Condoning assault? Such vile degradations demean our wives and daughters and corrupt America's face to the world."

On the tape, Trump, a 70-year-old one-time television reality show host, said, "When you're a star, they let you do it. You can do anything," and described how he would grab women's genitals.

FILE - YouTube screen grab from video obtained by The Washington Post of lewd conversation about women between Donald Trump and Billy Bush.

His crude language left newspapers and media outlets across the U.S. grappling with how to depict his comments, with some printing his words verbatim and others using a variety of euphemisms.

The disclosures come on top of long-known Trump slurs against women the Clinton campaign has collected in several ads for her candidacy.

Trump's polling numbers have dropped following his first debate performance with Clinton in late September, in which she kept him on the defensive for his business practices, his refusal to release his U.S. tax returns, and his long support for the debunked theory that Obama was born in Kenya and is not a U.S. citizen.

Since then, The New York Times disclosed three pages from his 1995 returns showing that he claimed $916 million in business losses that year, a deduction of such staggering proportions that he could have legally avoided paying income taxes for an 18-year stretch. Trump called his tax avoidance "brilliant."